Display devices are used to view images produced by digital processing devices such as desktop computers, laptop computers, televisions, mobile phones, smart phones, tablet computers, digital cameras, and other devices. A wide variety of technologies including cathode-ray tubes (CRTs), liquid crystal displays (LCDs), plasma display panels, and organic light emitting diodes (OLEDs) are used to implement display devices. Consequently, different display devices are able to represent colors within different gamuts. As used herein, the term “gamut” refers to a complete subset of colors that can be accurately represented by a particular display device. The gamuts for two different display devices have the following three possible relationships: (1) gamut 1 is larger than gamut 2, e.g., some colors that can be displayed in device 1 cannot be displayed in device 2, (2) gamut 1 is smaller than gamut 2, e.g., all colors that can be displayed in device 1 can also be displayed in device 2, and (3) gamut 1 partially overlaps with gamut 2.
Furthermore, the same color, as perceived by the human eye, might be represented by different numerical values in different gamuts. For example, the RGB color system is commonly used in computer graphics to represent colors of pixels in images. The same color might be represented by different RGB values in different gamuts. Consequently, gamut mapping is used to map color values between different gamuts so that the perceived colors generated using the color values might be the same in different devices. However, the RGB color system is not perceptually linear so that changes in the colors perceived by the human visual system are not proportional to changes in the RGB values. Other commonly used color systems including the HLS, HSV, and YIQ color systems are also perceptually non-linear. At least in part because of the perceptual nonlinearity of color systems, gamut mapping is difficult to perform in perceptually nonlinear color systems.
Gamut mapping is more straightforward in color systems that are perceptually uniform. As used herein, the phrase “perceptually uniform” refers to a color system in which uniform changes in the components of the color space defined by the color system correspond to uniform changes in perceived color. Relative perceptual differences between colors in a perceptually uniform color system are approximated by treating each color as a point in a three-dimensional (3-D) space and taking the Euclidean distance between the points that represent the two colors. For example, the CIELAB color system is almost perceptually uniform. There are other advanced color systems, such as CIECAM02, which are even more perceptually uniform than CIELAB. Gamut mapping of perceptually non-linear color systems (such as RGB) can therefore be performed by transforming the color values from the perceptually non-linear color system to a perceptually uniform color system (such as CIELAB) and then performing gamut mapping in the perceptually uniform color system. Gamut mapped values of the pixels are then transformed from the perceptually uniform color system back to the perceptually nonlinear color system (such as RGB). Gamut mapping by transformation into perceptually uniform color systems therefore incurs significant computational overhead.